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Full text of "The Cambridge Natural History"

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some others, very much subdivided in Rodents and other groups*
The degree of subdivision and the proportions of the several lobes
frequently offer valuable systematic characters. The gall-bladder
may be present or absent ; it is always a diverticulum of the
hepatic duct. The two are never separate, as in birds, for
Organs of Circulation.  The heart of all mammals is a com-
pletely four-chambered organ. In the adult heart there is no
communication between the right and left halves. The auricles
are comparatively thin- walled, the ventricles thick -walled, in
relation to the amount of work that they have severally to per-
form. The right ventricle, moreover, which has only to drive
the blood into the lungs, is much thinner-walled than the left
ventricle, which is concerned with the entire systemic circulation.
The exits of the arteries and the auriculo- ventricular orifices are
guarded by valves, which are so arranged as only to permit the
blood to now in the proper direction. But these valves have a
morphological as well as a physiological interest. At the origin
of each artery, the aorta and the pulmonary, there is a row of
three watch-pocket valves, as they have been generally termed on
account of their form. These three valves meet accurately in
the middle of the lumen of the arterial tube when liquid is
poured into them from above, and thus completely occlude the
orifice. The auriculo-ventricular valves differ in structure in the
two ventricles. That of the left ventricle has only two flaps,
and is therefore often spoken of as the bicuspid or mitral valve.
Both these flaps are membranous, and together they completely
surround the exit from the auricle into the ventricle. The edges
of the valve are bound down to the parietes of the heart by
numerous branching tendinous threads, the chordae tendineae,
which often take their origin from pillar-like muscles arising
from the walls of the heart, the so-called musculi papilla res. The
valve of the right ventricle is composed of three flaps, and is
therefore often spoken of as the tricuspid valve ; it is in the same
way membranous, and has chordae tendineae and musculi
papillares connected with it. The disposition of the musculi
papillares and their number differ in different mammals, but no
exhaustive study has as yet been made of the arrangements in
different groups ; the amount of individual variation even is not
known, though it is certainly considerable in some cases, for, in-
YOL. x